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POLisp: can a macro be recursive?
 

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  1. POLisp: can a macro be recursive?
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    <p>I've recently started coding in Lisp, and have already been most impressed with macros - they <a href="https://www.thanassis.space/score4.html#lisp" rel="nofollow noreferrer">allowed me to do complex loop-unrolling</a> at compile-time, something I can't do this elegantly in any other language that I know of (i.e. code-generate while keeping the original structure).</p> <p>On to optimization: I've sprinkled type annotations (lots of "the fixnum") in the same code. Once I've added 3 or 4 of them, I realized I was doing it wrong - this is what macros are for, Dont Repeat Yourself...</p> <pre><code>; whenever we want to indicate that the result of an operation ; fits in a fixnum, we macro expand (the fixnum (...)) (defmacro fast (&amp;rest args) `(the fixnum ,args)) ... (cond (...) (t (let* ((forOrange (+ (aref counts 5) (fast * 2 (aref counts 6)) (fast * 5 (aref counts 7)) (fast * 10 (aref counts 8)))) (forYellow (+ (aref counts 3) (fast * 2 (aref counts 2)) (fast * 5 (aref counts 1)) (fast * 10 (aref counts 0)))) </code></pre> <p>...and indeed, this worked: instead of writing lots of "(the fixnum (...))" everywhere, I just quickly prefix the expression with "fast" - and all is well.</p> <p>But then...</p> <p>I realized that even this is not where things should stop: in principle, the macro "fast" should... be called at the TOP of the evaluation, in this case:</p> <pre><code> (forYellow (fast + (aref counts 3) (* 2 (aref counts 2)) (* 5 (aref counts 1)) (* 10 (aref counts 0)))) </code></pre> <p>...and it should recursively "plant" "(the fixnum (...))" in all subexpressions.</p> <p>Can this be done? Can a "defmacro" be recursive?</p> <p><strong>UPDATE</strong>: I faced some really weird problems trying to do this, so I ended up doing what Rord suggested below - i.e. implemented a function, tested it in the repl, and calling it from the macro:</p> <pre><code>(defun operation-p (x) (or (equal x '+) (equal x '-) (equal x '*) (equal x '/))) (defun clone (sexpr) (cond ((listp sexpr) (if (null sexpr) () (let ((hd (car sexpr)) (tl (cdr sexpr))) (cond ((listp hd) (append (list (clone hd)) (clone tl))) ((operation-p hd) (list 'the 'fixnum (cons hd (clone tl)))) (t (cons hd (clone tl))))))) (t sexpr))) (defmacro fast (&amp;rest sexpr) `(,@(clone sexpr))) </code></pre> <p>And it works fine under SBCL:</p> <pre><code>$ sbcl This is SBCL 1.0.52, an implementation of ANSI Common Lisp. ... * (load "score4.cl") T * (setf a '(+ (1 2) (- 1 (+ 5 6))) ... * (clone a) (THE FIXNUM (+ (1 2) (THE FIXNUM (- 1 (THE FIXNUM (+ 5 6)))))) * (macroexpand '(fast + 1 2 THE FIXNUM (- 1 THE FIXNUM (+ 5 6)))) (THE FIXNUM (+ 1 2 THE FIXNUM (THE FIXNUM (- 1 THE FIXNUM (THE FIXNUM (+ 5 6)))))) T </code></pre> <p>All is well, except for one side-effect: CMUCL works, but no longer compiles the code:</p> <pre><code>; Error: (during macroexpansion) ; Error in KERNEL:%COERCE-TO-FUNCTION: the function CLONE is undefined. </code></pre> <p>Oh well :-)</p> <p><strong>UPDATE</strong>: The compilation failure was addressed and solved in <a href="https://stackoverflow.com/questions/8138714/solved-lisp-macro-calling-a-function-works-in-interpreter-fails-in-compiler">a different SO question</a>.</p>
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      1. POLisp: can a macro be recursive?
      1. USEmre Sevinç
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    1. COA style suggestion: Don't splice the form to be transformed into the macro call. Keep it as a subform, i.e. prefer `(fast (* ...))` over `(fast * ...)`. It makes the semantics simpler (consider the case `(fast 3)`—what is the meaning of this form supposed to be?) and caters to the reader's expectations about the structure of forms.
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      1. POLisp: can a macro be recursive?
      1. USMatthias Benkard
    2. CO@Matthias: Initially I had it like that - but I realized that in this case there are unambiguous semantics (i.e. "(fast 3)" can only mean "3") and... it makes it easier to use: you see a place in the code you want casts to fixnum, you don't have to hunt for paren-matching: just type "fast" in front.
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    3. COThis is quite common in Scheme.
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